This invention concerns the safety of moving industrial machines. There is a continuing need to provide safety to workers using industrial machines. Particularly in manufacturing apparatuses it is impractical to place all moving parts behind permanent barriers. Thus various techniques have evolved that tend to force safe operation of these machines.
One technique used in industrial machines requires both hands of each operator associated with the operation of a machine to operate the machine. Generally each operator must simultaneously press two push buttons to start operation of the machine. These two push buttons are located far enough apart that the operator is forced to use both hands. Release of either of these two push buttons during the closing operation of the machine stops the drive mechanism. Every push button is positioned so far from the machine that the closing operation is stopped before any of the operators' hands can reach a region in the machine where a hand could be trapped or crushed.
An additional technique generally used in industrial machines is a light curtain. A light curtain consists of plural light beams disposed to cover an extended area. These light beams are closely spaced so that a hand cannot pass through the protected zone without interrupting at least one of these light beams. Interruption of any one of these light beams initiates the stopping of the machine. This light curtain is disposed at a distance from any dangerous moving parts so that an operator's hand cannot breach the light curtain and reach a dangerous moving part before operation halts.
When such techniques are practiced, it is critical that the protective device be positioned far enough away from the dangerous moving parts of the machine so that the machine has time to decelerate to a stop before a hand can reach any moving parts. The position is based on the distance an operator's hand can move during the time required to completely stop the machine. There are standards for hand speed used in the computation of the minimum distance between the protective device and the dangerous moving part. Such computation requires an accurate determination of the time between triggering the protective device and the time the machine stops. Reliable measurement of this time period is often difficult.
A special purpose measurement apparatus is known in the industrial machine art. This measurement apparatus measures the time between the release of one of the two push buttons and the stopping of the machine. An electro-mechanical device is placed over one of the plurality of push buttons, holding the button down. Upon receipt of a trigger signal, the electro-mechanical device releases the push button simulating its release by the operator. The release of the push button will initiate the stopping of the machine. This trigger signal also starts a timer. The machine motion is measured using a tachometer attached to the machine via a spring wound cord. While the machine is moving, the tachometer produces an electrical output signal whose amplitude corresponds to the machine speed. Machine motion is adjudged stopped when the output signal decreases below a certain level. The timer is stopped when the machine has come to a complete stop. Such an apparatus provides an accurate, repeatable and reliable indication of the elapsed time required to completely stop the machine. This elapsed time and the standard hand speed determines the minimum safe distance between the two push buttons and any danger zone of the machine. Such an apparatus provides an accurate, repeatable and reliable indication of the duration of the machine's stopping interval.
Such a reliable and recognized apparatus is not known in the light curtain art. The current state of the art uses various ad hoc methods to determine the elapsed time to stop the dangerous moving parts. These methods are not generally recognized as reliable and repeatable, not applicable to the variety of industrial processes protected by light curtains or are expensive and time consuming. Thus the art needs an apparatus that can quickly and inexpensively provide an accurate, repeatable and reliable indication of duration of the stopping interval of these machines. Such an indication of the stopping interval can then be used with a standard hand speed to determine the correct placement of the light curtain.